The coal gasification process involves turning coal or other carbon-containing solids into synthesis gas. While both dry coal and water slurry can be used in the gasification process, dry coal pumping is more thermally efficient than current water slurry technology. For example, dry coal gasifiers have a thermal cold gas efficiency of approximately 82%, compared to water slurry gasifiers, which have a thermal cold gas efficiency of between approximately 70% and approximately 77%.
One of the devices currently being used to pump dry coal to a high pressure is the cycling lock hopper. While the thermal cold gas efficiency of cycling lock hopper fed gasifiers is higher than other currently available technology in the gasification field, the mechanical efficiency of the cycling lock hopper is relatively low, approximately 30%. The capital costs and operating costs of cycling lock hoppers are also high due to the high pressure tanks, valves, and gas compressors required in the cycling lock hopper process. Additionally, due to the complexity of the process and the frequency of equipment replacement required, the availability of the cycling lock hopper is also limited. Availability refers to the amount of time the equipment is on-line making product as well as to the performance of the equipment.
In order to simplify the process and increase the mechanical efficiency of dry coal gasification, the use of dry coal extrusion pumps has steadily become more common in dry coal gasification. Some of the problems associated with currently available dry coal extrusion pumps are internal shear failure zones and flow stagnation problems. The presence of failure zones can lead to a decreased mechanical efficiency in the pump. Some proposed solutions to internal shear failure zones and flow stagnation problems are to increase the pump flow rate and to use a linear or axial flow field geometry, rather than a cylindrical solids flow field geometry. While these solutions may increase the mechanical efficiency of the dry coal extrusion pump, other problems still persist.